At a guess, I'd say it's an early T24, maybe a demo run before it was decided to include it in the catalogue, so I'll say 2001.
Needing to call it something, they used a decal from the XB series that was coming to an end. Would not be the first time that Washburn used a strange decal.
The control-cavity cover is certainly different, & the tuning heads aren't tilted like the current bass.
The number you give makes no sense whatever. If it does indeed come from the company, then I'd say it's a demo piece, whether for early sales or a catalogue shot or adsheets. Even if that proves true, there's no way to tell whether it's unique or one of a hundred, so no guesses at rarity.
And some companies (G&L, among others) make demo pieces in order to test out different woods or glues. They might sell the result cheaply to a worker or a friendly store, but marked so that it can't be "returned" to some unsuspecting dealer for cash, or fixed under warranty.